A granulocyte microdroplet dye exclusion test was used to test granulocyte transfusion recipients prior to and after graulocyte transfusions. Thirty seven granulocyte transfusion recipients were tested prior to and following transfusion. Granulocytotoxins (GCT) were found in 24% initially and 45% after transfusion. The positive tests however did not correlate with transfusion reactions or more importantly with survival of the severely neutropenic patient. Dithiothreitol treated serum lost most of its activity suggesting that immunization to granulocyte antigens involves an IgM antibody response. Cryopreservation of granulocytes using standard published methods showed that granulocytes could be frozen and thawed and exclude trypan blue. The production of superoxide was not altered by the freeze thaw procedure but was severely depressed by the freezing solution. Studies to determine the offending agent present in the freezing solution are in progress since the cryoprotective agent DSMO is probably not the cause.